In many fields of the chemical industry there is a need for dehydration. The most important among them is the continuous dehydration of reaction mixture originating from reactions resulting in the liberation of water. The most characteristic representatives of the condensation reaction are direct esterification (the reaction of an organic acid or acid anhydride and an alcohol), direct amidation (the reaction of an organic acid or acid anhydride and an amine) and formation of an acetal or ketal (the reaction of an aldehyde or ketone and an alcohol). The continuous removal of water from the reaction mixture during progression of these kinds of reactions pushes the reaction equilibrium in the direction of product formation.
On an industrial scale water is continuously removed from the reaction mixture by extraction or by azeotropic distillation. The essence of the extraction is to add an organic solvent, which practically does not mix with water, to the reaction mixture, this way the water forming during the reaction separates in a new phase from the reaction medium. This manner is reported in the literature, c. f. e. g. J. Am. Chem. Soc., 70, 3135 (1948) and J. Pharm. Pharmacol. 2, 119 (1950). The removal of water by azeotropic distillation is carried out in such a way that an organic solvent, which forms an azeotropic mixture of minimal boiling point with water but does not mix with water, is added to the reaction mixture, and the vapors forming during boiling of the reaction mixture are condensed separately from the reaction zone, and from the two phases separating from the distillate the one more rich in the organic solvent, containing just a small amount of water, is recirculated to the reaction mixture.
A batch-wise azeotropic distillation method for the removal of water is described in the German patent specification No. 3,335,312. According to this method, organic solvents freely mixing with water are used. The disadvantage of the method is that the regeneration of these solvents is difficult, a rectification column is needed for the dehydration of the solvent. This makes the procedure complicated and expensive.
Water removal carried out by azeotropic distillation was also carried out using organic solvents both lighter and heavier than water. From among the solvents lighter than water, benzene, toluene and diisopropyl ether can be used advantageously (c.f.: Deak, Gy.: "Szerves vegyipari alapfolyamatok kezikonyve" (Handbook of the Basic Processes of Organic Chemistry--in Hungarian), 455 (1978), and DEA-29,17,087). Also many examples are known from the literature for the application of organic solvents heavier than water, for example carbon tetrachloride or 1,2-di-chloroethane (Org. Synt. Coll. Vol. I. 261 (1955and the U.S. Pat. No. 2,010,426).
Water removal carried out by azeotropic distillation is used still for dehydration of substances and mixtures (Kirk-Othmer: Encyclopedia of Chemical Technology, Azeotrop Distillation; III. Edition, Wiley-Interscience (1976)).
The disadvantage of the azeotropic distillation methods described above is that the water resulting from the reaction cannot be removed to such an extent that the given equilibrium reaction is complete from the point of view of the product.
Compared to the azeotropic distillation method, the extractive method is still less advantageous, because the extent of water removal reached is even lower than that reached by the known azeotropic distillation methods.
The most frequent application area of water removal, carried out by azeotropic distillation is direct esterification. From the point of view of water removal, in this case--and in the case of direct acetal and ketal forming--one more disadvantageous factor occurs.
In the reactions mentioned above often the 3-5-fold excess of such alcohols of short carbon chain, mostly of 1-4 carbon atoms is used as reactant, the boiling point of which is close to the boiling point of the solvent forming an azeotropic mixture of minimal boiling point with water, and practically not mixing with water. Because of the close value of the boiling points, a significant amount of the alcohol distills over with the azeotropic mixture, and this results in a significant dissolution between the separating phases in the condensate.
In many cases there is a need for a higher degree of dehydration than that attainable by using the known processes. This task occurs mostly at those equilibrium reactions, in which the value of the equilibrium constant is around one, or the equilibrium is shifted towards the hydrolysis, that is, opposite to the desired reaction. This is the situation, for example, in the esterification of piruvic acid, oxalic acid, ethylmalonic acid or phthalic acid, and the amidation of formic acid. In this case dehydration is carried out by recirculating the refluxing organic phase, removed from the reaction mixture by distillation, through a solid desiccant, e.g. CaCl.sub.2, P.sub.2 O.sub.5, K.sub.2 CO.sub.3, back to the reaction mixture (Org. Synth. Coll. Vol. I., 261 (1955)).
According to the method described in J. Org. Chem. 48, 3106 (1983) the reaction mixture is passed through a column filled with a solid desiccant and the low water concentration necessary for the favorable chemical transformation is produced this way.
On an industrial scale, the application of solid desiccants has the disadvantage that a huge amount must be used, and the regeneration of the desiccant is difficult, too. These factors make the realization of the reaction difficult and expensive.